The Magneto Effect

When writers give a villain a completely sound motivation/or just makes them right and then is forced to make them do something horrible to get you to not sympathize with them.

How do you make a Holocaust survivor who doesn’t want his people genocided AGAIN, into a villain? The Magneto Effect.

Humanity: Builds weapons of mutant genocide
Magneto: I don’t like that

Humanity: What a monster!?!?